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Zeotropic mixture
Zeotropic mixture
Zeotropic mixture
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A zeotropic mixture, or non-azeotropic mixture, is a mixture with liquid components that have different boiling points.[1] For example, a mixture of nitrogen, methane, ethane, propane, and isobutane constitutes a zeotropic mixture.[2] Individual substances within the mixture do not evaporate or condense at the same temperature as one substance. In other words, the mixture has a temperature glide, as the phase change occurs in a temperature range of about four to seven degrees Celsius, rather than at a constant temperature.[3] On temperature-composition graphs, this temperature glide can be seen as the temperature difference between the bubble point and dew point.[4] For zeotropic mixtures, the temperatures on the bubble (boiling) curve are between the individual component's boiling temperatures. When a zeotropic mixture is boiled or condensed, the composition of the liquid and the vapor changes according to the mixtures's temperature-composition diagram.[5] Zeotropic mixtures have different characteristics in nucleate and convective boiling, as well as in the organic Rankine cycle. Because zeotropic mixtures have different properties than pure fluids or azeotropic mixtures, zeotropic mixtures have many unique applications in industry, namely in distillation, refrigeration, and cleaning processes.

Dew and bubble points

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Figure 1: Temperature-Composition diagram of a zeotropic mixture

In mixtures of substances, the bubble point is the saturated liquid temperature, whereas the saturated vapor temperature is called the dew point. Because the bubble and dew lines of a zeotropic mixture's temperature-composition diagram do not intersect, a zeotropic mixture in its liquid phase has a different fraction of a component than the gas phase of the mixture. On a temperature-composition diagram, after a mixture in its liquid phase is heated to the temperature at the bubble (boiling) curve, the fraction of a component in the mixture changes along an isothermal line connecting the dew curve to the boiling curve as the mixture boils.[4]

At any given temperature, the composition of the liquid is the composition at the bubble point, whereas the composition of the vapor is the composition at the dew point.[5] Unlike azeotropic mixtures, there is no azeotropic point at any temperature on the diagram where the bubble line and dew lines would intersect.[4] Thus, the composition of the mixture will always change between the bubble and dew point component fractions upon boiling from a liquid to a gas until the mass fraction of a component reaches 1 (i.e. the zeotropic mixture is completely separated into its pure components). As shown in Figure 1, the mole fraction of component 1 decreases from 0.4 to around 0.15 as the liquid mixture boils to the gas phase.

Temperature glides

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Different zeotropic mixtures have different temperature glides. For example, zeotropic mixture R152a/R245fa has a higher temperature glide than R21/R245fa.[6] A larger gap between the boiling points creates a larger temperature glide between the boiling curve and dew curve at a given mass fraction. However, with any zeotropic mixture, the temperature glide decreases when the mass fraction of a component approaches 1 or 0 (i.e. when the mixture is almost separated into its pure components) because the boiling and dew curves get closer near these mass fractions.[4]

A larger difference in boiling points between the substances also affects the dew and bubble curves of the graph. A larger difference in boiling points creates a larger shift in mass fractions when the mixture boils at a given temperature.[4]

Zeotropic vs. azeotropic mixtures

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Figure 2: Bubble and Dew Curves for Zeotropic Mixtures

Azeotropic and zeotropic mixtures have different dew and bubble curves characteristics in a temperature-composition graph. Namely, azeotropic mixtures have dew and bubble curves that intersect, but zeotropic mixtures do not. In other words, zeotropic mixtures have no azeotropic points.[4] An azeotropic mixture that is near its azeotropic point has negligible zeotropic behavior and is near-azeotropic rather than zeotropic.[5]

Zeotropic mixtures differ from azeotropic mixtures in that the vapor and liquid phases of an azeotropic mixture have the same fraction of constituents. This is due to the constant boiling point of the azeotropic mixture.[7]

Boiling

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When superheating a substance, nucleate pool boiling and convective flow boiling occur when the temperature of the surface used to heat a liquid is higher than the liquid's boiling point by the wall superheat.[8]

Nucleate pool boiling

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The characteristics of pool boiling are different for zeotropic mixtures than that of pure mixtures. For example, the minimum superheating needed to achieve this boiling is greater for zeotropic mixtures than for pure liquids because of the different proportions of individual substances in the liquid versus gas phases of the zeotropic mixture. Zeotropic mixtures and pure liquids also have different critical heat fluxes. In addition, the heat transfer coefficients of zeotropic mixtures are less than the ideal values predicted using the coefficients of pure liquids. This decrease in heat transfer is due to the fact that the heat transfer coefficients of zeotropic mixtures do not increase proportionately with the mass fractions of the mixture's components.[9]

Convective flow boiling

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Zeotropic mixtures have different characteristics in convective boiling than pure substances or azeotropic mixtures. Overall, zeotropic mixtures transfer heat more efficiently at the bottom of the fluid, whereas pure and azeotropic substances transfer heat better at the top. During convective flow boiling, the thickness of the liquid film is less at the top of the film than at the bottom because of gravity. In the case of pure liquids and azeotropic mixtures, this decrease in thickness causes a decrease in the resistance to heat transfer. Thus, more heat is transferred and the heat transfer coefficient is higher at the top of the film. The opposite occurs for zeotropic mixtures. The decrease in film thickness near the top causes the component in the mixture with the higher boiling point to decrease in mass fraction. Thus, the resistance to mass transfer increases near the top of the liquid. Less heat is transferred, and the heat transfer coefficient is lower than at the bottom of the liquid film. Because the bottom of the liquid transfers heat better, it requires a lower wall temperature near the bottom than at the top to boil the zeotropic mixture.[9]

Heat transfer coefficient

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From low cryogenic to room temperatures, the heat transfer coefficients of zeotropic mixtures are sensitive to the mixture's composition, the diameter of the boiling tube, heat and mass fluxes, and the roughness of the surface. In addition, diluting the zeotropic mixture reduces the heat transfer coefficient. Decreasing the pressure when boiling the mixture only increases the coefficient slightly.[2] Using grooved rather than smooth boiling tubes increases the heat transfer coefficient.[10]

Distillation

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Figure 3 Distillation Column. Feed mixture enters from the middle of the column. Low-boiling component collects in the top rectifying section, while high-boiling component collects in the bottom stripping section.

The ideal case of distillation uses zeotropic mixtures. Zeotropic fluid and gaseous mixtures can be separated by distillation due to the difference in boiling points between the component mixtures. This process involves the use of vertically-arranged distillation columns (see Figure 2).[11][12]

Distillation columns

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When separating zeotropic mixtures with three or greater liquid components, each distillation column removes only the lowest-boiling point component and the highest boiling point component. In other words, each column separates two components purely. If three substances are separated with a single column, the substance with the intermediate boiling point will not be purely separated,[why?] and a second column would be needed. To separate mixtures consisting of multiple substances, a sequence of distillation columns must be used. This multi-step distillation process is also called rectification.[11][12]

In each distillation column, pure components form at the top (rectifying section) and bottom (stripping section) of the column when the starting liquid (called feed composition) is released in the middle of the column. This is shown in Figure 2. At a certain temperature, the component with the lowest boiling point (called distillate or overhead fraction) vaporizes and collects at the top of the column, whereas the component with the highest boiling point (called bottoms or bottom fraction) collects at the bottom of the column. In a zeotropic mixture, where more than one component exists, individual components move relative to each other as vapor flows up and liquid falls down.[11]

The separation of mixtures can be seen in a concentration profile. In a concentration profile, the position of a vapor in the distillation column is plotted against the concentration of the vapor. The component with the highest boiling point has a max concentration at the bottom of the column, where the component with the lowest boiling point has a max concentration at the top of the column. The component with the intermediate boiling point has a max concentration in the middle of the distillation column. Because of how these mixtures separate, mixtures with greater than three substances require more than one distillation column to separate the components.[11]

Distillation configurations

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Many configurations can be used to separate mixtures into the same products, though some schemes are more efficient, and different column sequencings are used to achieve different needs. For example, a zeotropic mixture ABC can be first separated into A and BC before separating BC to B and C. On the other hand, mixture ABC can be first separated into AB and C, and AB can lastly be separated into A and B. These two configurations are sharp-split configurations in which the intermediate boiling substance does not contaminate each separation step. On the other hand, the mixture ABC could first be separated into AB and BC, and lastly split into A, B, and C in the same column. This is a non-sharp split configuration in which the substance with the intermediate boiling point is present in different mixtures after a separation step.[12]

Efficiency optimization

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When designing distillation processes for separating zeotropic mixtures, the sequencing of distillation columns is vital to saving energy and costs. In addition, other methods can be used to lower the energy or equipment costs required to distill zeotropic mixtures. This includes combining distillation columns, using side columns, combining main columns with side columns, and re-using waste heat for the system. After combining distillation columns, the amount of energy used is only that of one separated column rather than both columns combined. In addition, using side columns saves energy by preventing different columns from carrying out the same separation of mixtures. Combining main and side columns saves equipment costs by reducing the number of heat exchangers in the system. Re-using waste heat requires the amount of heat and temperature levels of the waste to match that of the heat needed. Thus, using waste heat requires changing the pressure inside evaporators and condensers of the distillation system in order to control the temperatures needed. [13]

Controlling the temperature levels in a part of a system is possible with pinch analysis.[how?][14] These energy-saving techniques have a wide application in industrial distillation of zeotropic mixtures: side columns have been used to refine crude oil, and combining main and side columns is increasingly used.[13]

Examples of zeotropic mixtures

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Examples of distillation for zeotropic mixtures can be found in industry. Refining crude oil is an example of multi-component distillation in industry that has been used for more than 75 years.[as of?] Crude oil is separated into five components with main and side columns in a sharp split configuration. In addition, ethylene is separated from methane and ethane for industrial purposes using multi-component distillation.[12]

Separating aromatic substances requires extractive distillation, for example, distilling a zeotropic mixture of benzene, toluene, and p-xylene.[12]

Refrigeration

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Zeotropic mixtures that are used in refrigeration are assigned a number in the 400 series to help identify its component and their proportions as a part of nomenclature. Whereas for azeotropic mixtures they are assigned a number in the 500 series. According to ASHRAE, refrigerants names start with 'R' followed by a series of numbers—400 series if it is zeotropic or 500 if it is azeotropic—followed by uppercase letters that denote the composition.[15]

Research has proposed using zeotropic mixtures as substitutes to halogenated refrigerants due to the harmful effects that hydrochlorofluorocarbons (HCFC) and chlorofluorocarbons (CFC) have on the ozone layer and global warming. Researchers have focused on using new mixtures that have the same properties as past refrigerants to phase out harmful halogenated substances, in accordance to the Montreal Protocol and Kyoto Protocol. For example, researchers found that zeotropic mixture R-404A can replace R-12, a CFC, in household refrigerators.[16] However, there are some technical difficulties in using zeotropic mixtures. This includes leakages, as well as the high temperature glide associated with substances of different boiling points, though the temperature glide can be matched to the temperature difference between the two refrigerants when exchanging heat to increase efficiency.[5] Replacing pure refrigerants with mixtures calls for more research on the environmental impact as well as the flammability and safety of refrigerant mixtures.[3]

Organic Rankine cycle

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In the Organic Rankine Cycle (ORC), zeotropic mixtures are more thermally efficient than pure fluids.[17] Due to their higher boiling points, zeotropic working fluids have higher net outputs of energy at the low temperatures of the Rankine Cycle than pure substances.[6] Zeotropic working fluids condense across a range of temperatures, allowing external heat exchangers to recover the heat of condensation as a heat source for the Rankine Cycle. The changing temperature of the zeotropic working fluid can be matched to that of the fluid being heated or cooled to save waste heat because the mixture's evaporation process occurs at a temperature glide[17][18] (see pinch analysis).

R21/R245fa and R152a/R245fa are two examples of zeotropic working fluids that can absorb more heat than pure R245fa due to their increased boiling points. The power output increases with the proportion of R152a in R152a/R245fa. R21/R245fa uses less heat and energy than R245fa. Overall, zeotropic mixture R21/R245fa has better thermodynamic properties than pure R245fa and R152a/R245fa as a working fluid in the ORC.[6][18]

Cleaning processes

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Zeotropic mixtures can be used as solvents in cleaning processes in manufacturing. Cleaning processes that use zeotropic mixtures include cosolvent processes and bisolvent processes.[19]

Cosolvent and bisolvent processes

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In a cosolvent system, two miscible fluids with different boiling points are mixed to create a zeotropic mixture. The first fluid is a solvating agent that dissolves soil in the cleaning process. This fluid is an organic solvent with a low-boiling point and a flash point greater than the system's operating temperature. After the solvent mixes with the oil, the second fluid, a hydrofluoroether rinsing agent (HFE), rinses off the solvating agent. The solvating agent can be flammable because its mixture with the HFE is nonflammable. In bisolvent cleaning processes, the rinsing agent is separated from the solvating agent. This makes the solvating and rinsing agents more effective because they are not diluted.[19][20]

Cosolvent systems are used for heavy oils, waxes, greases and fingerprints, and can remove heavier soils than processes that use pure or azeotropic solvents. Cosolvent systems are flexible in that different proportions of substances in the zeotropic mixture can be used to satisfy different cleaning purposes. For example, increasing the proportion of solvating agent to rinsing agent in the mixture increases the solvency, and thus is used for removing heavier soils.[19][20]

The operating temperature of the system depends on the boiling point of the mixture, which in turn depends on the compositions of these agents in zeotropic mixture. Since zeotropic mixtures have different boiling points, the cleaning and rinse sump have different ratios of cleaning and solvating agents. The lower-boiling point solvating agent is not found in the rinse sump due to the large difference in boiling points between the agents.[20]

Examples of zeotropic solvents

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Mixtures containing HFC-43-10mee can replace CFC-113 and perfluorocarbon (PFC) as solvents in cleaning systems because HFC-43-10mee does not harm the ozone layer, unlike CFC-113 and PFC. Various mixtures of HFC-43-10mee are commercially available for a variety of cleaning purposes. Examples of zeotropic solvents in cleaning processes include:[20]

  • Zeotropic mixtures of HFC-43-10mee and hexamethyldisiloxane can dissolve silicones and are highly compatible with polycarbonates and polyurethane. They can be used to remove silicone lubricant from medical devices.
  • Zeotropic mixtures of HFC-43-10mee and isopropanol can remove ions and water from materials without porous surfaces. This zeotropic mixture helps with absorption drying.
  • Zeotropic mixtures of HFC-43-10mee, fluorosurfactant, and antistatic additives are energy-efficient and environmentally safe drying fluids that provide spot-free drying.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Zeotropic mixture

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A zeotropic mixture, also known as a non-azeotropic mixture, is a blend of two or more components possessing distinct points, resulting in a phase change that occurs over a range of at constant rather than isothermally. This characteristic temperature glide—defined as the difference between the and temperatures during or —distinguishes zeotropic mixtures from azeotropic ones, where the vapor and liquid phases maintain identical compositions and exhibit negligible glide, mimicking the behavior of a pure substance. The glide typically ranges from 5 K to 50 K depending on composition and operating conditions, enabling more efficient by aligning the mixture's temperature profile with varying heat source or sink temperatures. In thermodynamic applications, zeotropic mixtures are widely employed in cycles, organic Rankine cycles (), and systems to enhance overall efficiency. For instance, in , these mixtures can improve the (COP) by up to 40% compared to pure fluids, primarily by reducing destruction in evaporators and condensers through better thermal matching. Similarly, in low-grade recovery via ORC, zeotropic blends like R1336mzz(Z)/R1336mzz(E) have demonstrated power-to-power efficiencies of 71.6% and ORC efficiencies up to 4.46%, outperforming pure refrigerants by 23% under optimal compositions. Common examples include hydrocarbon-CO₂ blends and mixtures, selected for their low (GWP) and compatibility with existing systems. Despite these benefits, zeotropic mixtures present challenges such as composition shifts during phase separation or leakage, which can alter performance, and the need for precise control to maximize the temperature glide's advantages. Research continues to optimize blend ratios for specific applications, focusing on thermodynamic properties like volumetric power coefficients and exergetic efficiencies to support systems.

Thermodynamic Properties

Dew and Bubble Points

The of a zeotropic is defined as the at which the first vapor bubble forms within the phase when the mixture is heated at a constant and fixed overall composition. This occurs as the sum of the partial pressures of the components equals the total system , marking the onset of . Conversely, the is the at which the first droplet condenses from the vapor phase when the mixture is cooled at a constant and fixed overall composition. At this point, the vapor is saturated, and further cooling leads to formation as the partial pressures allow to begin. In zeotropic mixtures, the and bubble points differ because the components exhibit varying volatilities, causing the liquid and vapor phases to have distinct compositions at equilibrium. This compositional shift arises as more volatile components preferentially enter the vapor phase, enriching it relative to the liquid, while less volatile components concentrate in the liquid. For ideal zeotropic mixtures, these phase equilibria can be mathematically represented using , which states that the partial pressure of each component ii in the liquid phase is given by pi=xiPi∘(T),p_i = x_i P_i^\circ(T), where xix_i is the liquid mole fraction of component ii, and Pi∘(T)P_i^\circ(T) is the saturation vapor pressure of pure component ii at temperature TT. At the bubble point, the total pressure PP equals the sum of all partial pressures: P=∑ipi=∑ixiPi∘(T),P = \sum_i p_i = \sum_i x_i P_i^\circ(T), and the vapor mole fraction yiy_i is calculated as yi=piP=xiPi∘(T)P.y_i = \frac{p_i}{P} = \frac{x_i P_i^\circ(T)}{P}. This framework allows determination of the bubble point temperature by solving for TT where the equation holds for given xix_i and PP. For the dew point, the relations are inverted, using vapor compositions yiy_i to find the liquid fractions via xi=yiP/Pi∘(T)x_i = y_i P / P_i^\circ(T), with ∑ixi=1\sum_i x_i = 1. For non-ideal zeotropic mixtures, deviations from Raoult's law require models incorporating activity coefficients or equations of state for precise calculations. Phase diagrams for zeotropic mixtures, typically plotted as temperature-composition (T-x-y) diagrams at constant pressure, illustrate these concepts by showing the curve (liquid composition xx vs. TT) and curve (vapor composition yy vs. TT). In such diagrams, the lines diverge across compositions, with the vapor curve lying above the curve, reflecting the enrichment of volatile components in the vapor phase and highlighting the non-constant behavior inherent to zeotropic systems.

Temperature Glides

In zeotropic mixtures, the temperature glide refers to the continuous variation in temperature that occurs during the phase change process—either or —at constant pressure, spanning from the to the . This phenomenon arises because the and vapor phases have different compositions, with the more volatile components enriching the vapor phase and the less volatile ones concentrating in the phase. For common hydrocarbon zeotropic mixtures, such as binary blends of and (R-290/R-600), the temperature glide can reach up to 12°C, depending on the composition and operating conditions; for instance, a 44% mass fraction yields a maximum of approximately 12°C at a of 60°C. Higher glides, up to 44°C, can occur in mixtures like and pentane (R-290/R-601) at optimized fractions around 35% . The glide impacts by altering the driving force in and condensers, often reducing the compared to pure fluids and leading to lower overall coefficients due to the non-uniform profile during phase change. This can decrease the effective rate unless the glide is matched to the source or profile, potentially improving cycle in some cases but complicating design. The magnitude of the temperature glide is calculated as the difference between the temperature (TdewT_{dew}) and the temperature (TbubbleT_{bubble}) at a given : ΔT=Tdew−Tbubble\Delta T = T_{dew} - T_{bubble} For a binary hydrocarbon mixture like (50% mass fraction) and at 5 bar , thermodynamic models yield a ΔT\Delta T of approximately 10–15°C. During phase change, the temperature glide induces , where the vapor phase becomes enriched with higher-volatility components (e.g., more in a propane-butane blend), while the phase concentrates the less volatile ones, potentially leading to composition shifts along the and uneven performance if not accounted for in system design.

Zeotropic vs. Azeotropic Mixtures

Azeotropic mixtures are defined as combinations of two or more s that boil at a constant and maintain the same composition in both the liquid and vapor phases during , behaving essentially like a single pure component. In contrast, zeotropic mixtures consist of components with differing boiling points, resulting in distinct compositions between the liquid and vapor phases at equilibrium, which prevents the formation of an azeotropic point. The primary difference lies in separability: zeotropic mixtures lack an azeotropic point, allowing effective separation through as the vapor phase is enriched in the more volatile component, progressively changing the overall composition. Azeotropic mixtures, however, cannot be separated by conventional because the phase compositions remain identical, limiting purification to the azeotropic composition. Thermodynamically, zeotropic mixtures exhibit behavior—either ideal or non-ideal—where the and curves do not intersect, avoiding minimum or maximum points that align with pure component temperatures, thus enabling composition-dependent phase changes. This contrasts with azeotropes, which arise from significant positive or negative deviations from , causing the phase curves to cross and fix the independent of further composition shifts. The behavior of zeotropic mixtures in systems was systematically utilized starting in the mid-19th century, as advancements in refining employed to separate complex crude oil blends into usable fractions like and . Practically, zeotropic mixtures can be readily fractionated using standard techniques, facilitating efficient component recovery in processes like processing, whereas azeotropic mixtures necessitate alternative methods such as extractive or with added entrainers to achieve separation. This ease of fractionation makes zeotropes advantageous for applications requiring purity, while azeotropes pose challenges in industries like chemical manufacturing, often increasing energy and equipment costs. Unlike azeotropes, zeotropic mixtures also feature unique temperature glides during phase change, further distinguishing their thermal profiles.

Boiling Behavior

Nucleate Pool Boiling

Nucleate pool boiling refers to the regime in which discrete bubbles nucleate, grow, and detach from specific sites on a heated surface immersed in a stagnant pool of liquid, facilitating enhanced through the agitation and transport associated with bubble dynamics. In zeotropic mixtures, this process is inherently more complex than in pure fluids due to the continuous variation in liquid and vapor compositions during phase change, leading to coupled and phenomena that degrade overall performance. Zeotropic mixtures require greater wall superheat to initiate and sustain compared to pure fluids, primarily because of resistance arising from composition gradients at the vapor-liquid interface. This resistance stems from the need for the less volatile component to diffuse toward the interface to replace the preferentially evaporated more volatile component, effectively increasing the local and necessitating higher differences for bubble formation. The glide inherent to zeotropic mixtures further exacerbates superheat requirements by introducing spatial and temporal variations in saturation during . During bubble departure in zeotropic mixtures, the more volatile component undergoes preferential within the growing bubble, resulting in a vapor phase enriched with this component while the adjacent becomes depleted. This compositional shift induces interfacial tension gradients and alters forces, typically leading to smaller bubble departure diameters and higher bubble release frequencies than observed in pure fluids, which helps maintain but at reduced efficiency due to the additional diffusion limitations. The heat flux in nucleate pool boiling for zeotropic mixtures is often predicted using an adapted version of the Rohsenow correlation, originally developed for pure fluids as q=μlhfg[g(ρl−ρv)σ]1/2(cp,lΔTCsfhfgPrln)3q = \mu_l h_{fg} \left[ \frac{g(\rho_l - \rho_v)}{\sigma} \right]^{1/2} \left( \frac{c_{p,l} \Delta T}{C_{sf} h_{fg} Pr_l^n} \right)^3 where qq is the heat flux, μl\mu_l the liquid viscosity, hfgh_{fg} the latent heat of vaporization, gg gravity, ρl\rho_l and ρv\rho_v the liquid and vapor densities, σ\sigma the surface tension, cp,lc_{p,l} the liquid specific heat, ΔT\Delta T the wall superheat, CsfC_{sf} a surface-fluid constant, and PrlPr_l the liquid Prandtl number with exponent nn. For mixtures, this model incorporates corrections such as degradation factors to account for mass transfer resistance and non-ideal mixture effects, improving predictive accuracy across various compositions. Experimental investigations reveal that the in nucleate pool boiling of zeotropic mixtures can be comparable to or higher than that of their pure components, influenced by composition-induced effects on bubble dynamics. For instance, in binary zeotropic systems like R134a/R245fa, studies show increased CHF values compared to pure components, attributed to enhanced bubble coalescence and stable interfacial conditions.

Convective Flow Boiling

Convective flow of zeotropic mixtures takes place in channels or , where the bulk flow velocity promotes bubble detachment from the heated surface, thereby enhancing compared to stagnant conditions. This process combines in the liquid phase with evaporation at the vapor-liquid interface, particularly dominant at higher mass fluxes and vapor qualities. In zeotropic mixtures, the glide introduces composition-dependent variations in thermophysical properties, influencing the overall dynamics. Typical flow regimes during convective boiling include bubbly flow at low vapor qualities, slug flow with elongated bubbles, and annular flow where a liquid film coats the tube wall with a vapor core. For zeotropic mixtures, the temperature glide promotes fractionation, with vapor phases enriched in more volatile components, which can accelerate the transition to annular flow by enhancing interfacial mass transfer and reducing bubbly regime persistence. This earlier shift to annular flow aids in maintaining efficient evaporation along the tube length. Two-phase pressure drop in convective flow boiling of zeotropic mixtures is commonly predicted using adaptations of the Lockhart-Martinelli correlation, which incorporates the Martinelli parameter to relate two-phase flow to single-phase friction while accounting for varying mixture compositions and properties. These adaptations, such as those evaluated by Jung and Radermacher, demonstrate good agreement with experimental data for mixed refrigerants by adjusting for slip ratios and void fractions influenced by the glide. Zeotropic working fluids provide efficiency advantages in the bottom sections of vertical evaporators, where the temperature glide aligns the fluid's saturation profile more closely with the heating medium, minimizing thermodynamic irreversibilities and improving overall cycle performance. This matching reduces losses, making zeotropes preferable for applications like organic Rankine cycles.

Heat Transfer Coefficient

The (HTC) quantifies the rate of heat transfer during of zeotropic mixtures and is defined as h=qΔTh = \frac{q}{\Delta T}, where qq is the and ΔT\Delta T is the wall superheat, typically the difference between the surface temperature and the local saturation . In zeotropic mixtures, the HTC is generally lower than that of pure fluids, with reductions ranging from 20% to 50% due to resistance and the inherent temperature glide, which leads to compositional variations between liquid and vapor phases. This degradation arises primarily from the temperature glide, which causes local mismatches in profiles within heat exchangers, suppressing contributions and altering the balance between mechanisms. Predictive models for HTC in zeotropic mixtures often adapt the Chen correlation, which decomposes the total HTC into nucleate boiling and convective components as h=hnb+hconvh = h_{nb} + h_{conv}, where suppression factors (e.g., SS) account for mixture effects on nucleate boiling and enhancement factors (e.g., FF) adjust for convective contributions. These models incorporate dimensionless parameters like the temperature glide ratio T∗=TgTsatT^* = \frac{T_g}{T_{sat}} to capture the impact of glide on suppression, enabling predictions with mean absolute percentage deviations (MAPD) as low as 24.6% for flow boiling in horizontal tubes. For instance, in mini-channel flow boiling of R290/R601a mixtures, the Chen model has been refined with mixture-specific corrections, achieving a mean absolute relative deviation (MARD) of 13.7%. Experimental trends indicate that the HTC in zeotropic mixtures typically peaks at intermediate vapor qualities (e.g., around 0.5), where the interplay of nucleate and convective is optimized before declining due to dryout or increased sensible heating at higher qualities. The HTC increases with rising (e.g., ~9.7% per 100 kg/m²·s increment) and but decreases with higher saturation pressure (e.g., ~9.4% from 1 to 1.5 MPa), with larger glides exacerbating these effects through enhanced limitations. In systems with large glides (30–38°C), such as CO₂/R152a, sensible heating can account for 13–15% of the total heat load, further reducing peak HTC values compared to pure components. Optimization strategies for HTC in zeotropic mixtures focus on matching the mixture's temperature glide to the required temperature lift in thermodynamic cycles, such as heat pumps, to minimize losses despite the inherently lower HTC. This approach enhances overall cycle efficiency by aligning phase-change temperatures with heat source/sink profiles, potentially improving (COP) while compensating for glide-induced mismatches in design.

Distillation Processes

Distillation Columns

Distillation columns for zeotropic mixtures exploit differences in component volatilities to achieve separation, as these mixtures lack azeotrope formation, enabling complete fractionation using conventional equipment. Tray columns, equipped with sieve or valve trays, provide discrete stages for vapor-liquid contact, promoting efficient mass transfer through bubbling and weeping mechanisms, while packed columns utilize random or structured packing to facilitate continuous contact with lower pressure drops, making both types suitable for zeotropic separations where relative volatilities remain favorable across compositions. The separation in these columns is driven by vapor-liquid equilibrium (VLE), where more volatile components preferentially enter the vapor phase, creating a composition gradient along the column height that enables countercurrent . For binary zeotropic mixtures, the minimum number of theoretical stages required at total can be estimated using the : Nmin⁡=log⁡[xD/(1−xD)xB/(1−xB)]log⁡αN_{\min} = \frac{\log\left[\frac{x_D/(1-x_D)}{x_B/(1-x_B)}\right]}{\log \alpha} where xDx_D and xBx_B are the distillate and bottoms compositions of the light key component, respectively, and α\alpha is the ; this equation assumes constant α\alpha and is applicable to zeotropes due to their non-constant behavior without pinching. Key operational parameters include the reflux ratio, which determines the liquid return to the column and influences separation sharpness by increasing internal flows, typically operated above the minimum for economic balance in both binary and multicomponent zeotropic systems, and the feed stage location, optimally selected near the feed composition's equilibrium stage to minimize stages and energy use. In multicomponent zeotropes, these parameters must account for distributed components, often prioritizing key separations while non-keys follow naturally. Scale-up from to industrial columns for zeotropic mixtures requires careful consideration of composition-dependent properties, such as varying , , and along the column, which impact flooding limits, tray hydraulics, and packing efficiency, necessitating pilot testing or to ensure stable operation at larger diameters and throughputs.

Distillation Configurations

For binary zeotropic mixtures, separation is typically achieved using a single distillation column, leveraging the continuous variation in composition between the liquid and vapor phases to achieve high-purity products at the distillate and bottoms. This configuration relies on the differences inherent in zeotropic systems, allowing straightforward without the complications of constant-boiling behavior. For multicomponent zeotropic mixtures with more than three components, multi-column configurations such as direct-sequential sequences are employed to systematically separate components by prioritizing the lightest or heaviest fractions first in successive columns. In a direct-sequential setup, the feed is initially processed to isolate the lightest component, with the bottoms stream advancing to subsequent columns for further splits, minimizing remixing and optimizing energy use across the sequence. These arrangements are particularly suited for or higher zeotropic systems, where the number of columns equals the number of components minus one in basic configurations. Heat-integrated designs, such as dividing wall columns, enhance efficiency in separating multicomponent zeotropic mixtures by incorporating a vertical partition within a single shell to perform multiple separations simultaneously, reducing by approximately 30% compared to conventional multi-column setups. This configuration eliminates the need for a split between intermediate columns, promoting better integration and lower capital costs while maintaining separation sharpness for zeotropic feeds like wide-boiling blends. Reactive distillation variants adapt the process for zeotropic mixtures in by integrating catalytic reactions within the column to shift equilibria and facilitate separation, as seen in esterification processes where zeotropic alcohol-acid feeds produce distillable ester-water products. These setups combine reaction zones in the lower sections with purification in upper trays, enhancing conversion for reversible reactions involving zeotropic intermediates without forming azeotropes. In fractionation, multi-column configurations process zeotropic mixtures from crude oil, starting with an atmospheric unit to separate light ends like and , followed by for heavier residues, enabling efficient recovery of fuels and lubricants from complex, non-ideal zeotropic feeds. This sequential approach handles the broad boiling range of hydrocarbons, yielding distinct fractions while minimizing energy through side-stream integrations.

Efficiency Optimization

Optimization of zeotropic distillation processes focuses on minimizing consumption and operational costs while achieving desired separation purity, particularly for multicomponent mixtures where non-ideal behaviors complicate heat duties. serves as a key thermodynamic tool to identify minimum requirements by constructing composite curves that highlight the pinch point—the where recovery is most constrained—allowing targeted reduction in utility costs such as and cooling water. This method has been applied to columns to achieve savings of up to 30% in heat-integrated designs by optimizing exchanger networks around the pinch . Variable policies enhance efficiency in of zeotropic mixtures by dynamically adjusting the reflux ratio to counteract composition shifts induced by glides, where the varying points of components lead to non-uniform vapor-liquid equilibria along the column. Unlike constant reflux operations, which result in fluctuating distillate purity, variable policies maintain optimal separation trajectories, reducing overall cycle time and input by 10-15% in ternary zeotropic systems. Advanced simulation tools like Aspen Plus are essential for modeling non-ideal zeotropic behaviors, incorporating activity coefficient models such as NRTL or to predict phase equilibria and column profiles accurately under varying pressures and compositions. These simulations facilitate parametric optimization of ratios, feed locations, and numbers, enabling rapid evaluation of energy-efficient designs without extensive experimentation. Hybrid distillation-membrane processes further boost efficiency for zeotropic separations, particularly close-boiling variants, by using membranes to selectively permeate one component post-distillation, reducing the need for excessive reflux and reboiler duty. A primary challenge in optimizing zeotropic distillation lies in handling wide boiling ranges between components, which necessitate taller columns with more theoretical stages and higher reflux ratios to achieve sharp separations, potentially increasing capital and energy costs by 20-50% if not addressed through heat integration. Certain column configurations, such as thermally coupled schemes, enable these optimizations by facilitating internal heat recovery across wide temperature spans.

Applications in Refrigeration and Power Cycles

Refrigeration Systems

Zeotropic mixtures, particularly non-azeotropic blends in the 400-series such as R-404A (composed of 44% R-125, 52% R-143a, and 4% R-134a), have been widely adopted as working fluids in vapor-compression refrigeration systems as replacements for ozone-depleting CFCs and HCFCs like R-12 and R-502. However, as of 2025, high-GWP blends like R-404A are being phased out in favor of low-GWP alternatives under international regulations such as the Kigali Amendment. These blends emerged in response to the 1987 Montreal Protocol, which mandated the phaseout of ozone-depleting substances, leading to their commercial adoption in the 1990s for commercial and industrial refrigeration applications. The primary advantage of zeotropic mixtures in these systems stems from their temperature glide, which allows the refrigerant's phase-change to vary during and at constant , better matching the temperature profiles of the and condenser heat exchangers. This glide matching reduces thermodynamic irreversibilities and losses, potentially improving the (COP) by 5-10% compared to single-component refrigerants in systems with finite rates. For instance, in air-cooled condensers or with varying external fluid temperatures, this leads to enhanced efficiency without requiring major redesigns. In cycle analysis, the glide influences key components: expansion valves must be sized based on the pressure to ensure proper superheat control, as the glide causes a gradual across the valve, potentially requiring adjustments to the expansion valve sizing compared to azeotropic fluids. work is also affected, with the varying suction gas during leading to slightly higher average compression ratios, though the overall cycle gains often offset this in optimized designs. The glide briefly enhances in evaporators by promoting more uniform differences, but system controls must account for this to avoid capacity fluctuations. A notable drawback is fractionation during leaks, where components with higher vapor pressures (e.g., R-134a in R-404A) escape preferentially, altering the mixture composition and glide, which can cause minor changes in performance if not recharged properly, though the impact is typically minimal for low-glide blends like R-404A. This necessitates glide-tolerant designs, such as electronic expansion valves and composition monitoring, to maintain performance over the system's lifecycle.

Organic Rankine Cycles

Organic Rankine cycles (ORCs) utilize zeotropic mixtures as working fluids to recover low-grade heat sources, such as those below 200°C, by leveraging the temperature glide during phase change to better match the heat source and sink profiles, thereby reducing thermal mismatches compared to pure fluids. For instance, the zeotropic blend R-245fa/R-152a enables a gliding evaporation and condensation process that aligns more closely with the variable temperatures of industrial waste heat or geothermal fluids, potentially boosting thermal efficiency by 15-20% over pure working fluids in subcritical cycles. This improvement stems from minimized exergy destruction in heat exchangers, as the non-isothermal heat transfer decreases the temperature difference driving irreversibilities. Working fluid selection for zeotropic ORCs prioritizes mixtures with higher bubble points to maintain subcritical operation under typical low-grade heat conditions, ensuring the dew point exceeds the heat sink temperature while providing sufficient glide for efficiency gains. Blends like R-245fa/R-152a are favored for their thermodynamic properties, including moderate critical temperatures and environmental compatibility, which allow operation without exceeding critical points in geothermal or applications. analysis further validates these selections, revealing reduced overall irreversibilities—often by 10-15%—due to the glide effect, which lowers generation in the and condenser compared to isobaric phase changes with pure fluids. Since the , zeotropic blends have been extensively studied and show potential for implementation in geothermal and recovery plants, with analyses demonstrating up to 20% higher second-law in systems operating at 100-180°C. For example, research on geothermal configurations in regions like and has explored mixtures such as R-245fa with hydrocarbons to optimize extraction from brines around 150°C, while heat recovery from flue gases in plants has utilized similar blends for net power increases of 5-10 kW per module in simulations. Integration challenges include adapting expander designs, such as single-screw or radial turbines, to handle the and variable density of zeotropic vapors during expansion, often requiring optimized inlet nozzles for 70-80% isentropic . Recuperators are similarly modified with enhanced surface areas or counterflow configurations to exploit the mixture's recovery potential, further improving cycle performance by 5-8%.

Industrial Cleaning Applications

Cosolvent and Bisolvant Processes

Cosolvent processes involve the formulation of zeotropic mixtures by blending miscible fluids with differing boiling points, such as non-halogenated solvents with hydrofluorocarbons, to achieve customized profiles and controlled rates tailored for removing contaminants from industrial surfaces. These blends leverage the inherent phase behavior of zeotropic mixtures, where the glide during —referring to the difference between and bubble points—facilitates gradual drying that minimizes residue on cleaned parts. Bisolvant systems extend this approach by employing zeotropic formulations that exhibit partial immiscibility post-cleaning, allowing the mixture to separate into distinct phases for straightforward recovery and . This separation mechanism enhances process efficiency by enabling or of the components, reducing the need for complex purification steps and supporting sustainable operations in high-volume . The adoption of zeotropic mixtures in cosolvent and bisolvant processes yields environmental advantages, notably lower VOC emissions relative to conventional pure solvents, as the tunable and recovery features limit atmospheric release during application and . In practice, these systems are applied via spray or immersion methods, optimized for sectors like electronics manufacturing where parameters such as fluid pressure, exposure duration, and bath temperature are adjusted to balance with component .

Examples of Zeotropic Solvents

One practical example of a zeotropic mixture is blends such as those containing 75-99 wt% HFC-43-10mee (1,1,1,2,3,4,4,5,5,5-decafluoropentane) with 0.1-5 wt% isopropanol (often including additional co-solvents like HFC-365mfc), designed for precision cleaning of sensitive electronic and mechanical components. This non-azeotropic formulation combines the mild solvency and low of HFC-43-10mee with the enhanced cleaning power of isopropanol to effectively remove ionic residues, oils, and without damaging non-porous surfaces. Another example is n-propyl bromide (nPB) and blends, used for robust and residue removal in vapor processes. These mixtures leverage nPB's strong with 's polarity to dissolve polar and non-polar contaminants like fluxes and greases, making them suitable for industrial-scale cleaning. These zeotropic solvents demonstrate high performance in cleaning tasks, with solvency indices such as Kauri-Butanol (KB) values exceeding 90—reaching up to 130 for nPB-based blends—enabling rapid dissolution of stubborn soils compared to milder fluorinated solvents alone. The inherent temperature glide of 2-5°C in these non-azeotropic mixtures facilitates faster during evaporation in vapor by allowing progressive phase changes that minimize residue and improve throughput. In industry applications, HFC-43-10mee/isopropanol blends are employed for degreasing components, such as system parts, to meet stringent standards for and reliability. Similarly, nPB/ blends are used in (PCB) cleaning to remove fluxes and ionic contaminants, ensuring electrical performance in . However, as of 2025, EPA regulations are restricting nPB use in many applications due to health risks. Safety considerations for these mixtures include non-flammability for HFC-43-10mee/isopropanol blends (classified as non-flammable under ASTM D56), with low (LC50 > 100,000 ppm) and no , though ventilation is recommended to limit exposure below 200 ppm. nPB/ethanol blends carry flammability risks ( around 22°C) and potential , prompting California PEL of 5 ppm (8-hour ); federal OSHA has no PEL, with NIOSH recommending 0.1 ppm (10-hour ) and requirements for stabilization to prevent . Recent formulations in the 2020s feature emerging bio-based zeotropic solvents, such as blends of soy methyl esters with or , offering sustainable alternatives for precision cleaning with reduced environmental impact and comparable solvency to traditional options. These renewable mixtures support processes by providing tunable glides for efficient drying while meeting low-GWP and biodegradability goals in industrial applications. These developments align with 2024-2025 EPA regulations restricting toxic solvents like nPB, promoting bio-based alternatives for sustainable cleaning.

Modern Developments

Low-GWP Refrigerants

The to the , adopted in 2016, initiated a global phase-down of hydrofluorocarbons (HFCs) to mitigate their contribution to , prompting the development of (HFO)-based zeotropic refrigerant blends as low-global-warming-potential (GWP) alternatives. These blends, such as (68.9% R-32 and 31.1% R-1234yf by weight), exhibit a GWP of 466, representing a 78% reduction compared to the traditional 400-series refrigerant (GWP 2088). For applications requiring even lower GWPs under stricter limits, blends like R-454C (21.5% R-32 and 78.5% R-1234yf) achieve a GWP of approximately 148. Blend design for these HFO-based zeotropes focuses on optimizing thermodynamic glide—typically 1-7°C depending on composition—to serve as drop-in replacements for in and systems, enhancing efficiency during phase change without major equipment redesign. tests demonstrate that such blends retain 95-103% of R-410A's (COP) while enabling up to 20% reduced refrigerant charge due to improved volumetric efficiency and lower pressures. Regulatory frameworks, including the EU F-Gas Regulation, mandate GWP limits below 150 for new hermetically sealed and systems starting January 1, 2025, accelerating adoption of these zeotropic mixtures to comply with environmental standards. However, their mild flammability ( A2L classification) necessitates safety measures, such as sensors and component modifications, to mitigate ignition risks in residential and commercial installations.

Emerging Uses in Carbon Capture

Zeotropic mixtures, particularly amine blends such as monoethanolamine (MEA) and N,N-diethylethanolamine (DEEA), play a significant role in absorption-based carbon capture processes by leveraging their non-ideal vapor-liquid equilibrium properties, including temperature glides, to enhance CO2 selectivity and absorption kinetics. These blends exhibit improved coefficients compared to pure , allowing for higher CO2 loading capacities and reduced circulation rates in post-combustion capture systems. For instance, aqueous MEA/DEEA solutions demonstrate superior overall CO2 performance due to synergistic effects between the primary (MEA) for fast reaction kinetics and the tertiary (DEEA) for bulk CO2 absorption, resulting in enhanced selectivity over other components like SOx and . Pilot-scale demonstrations in the have highlighted gains from zeotropic mixtures, achieving approximately 20% lower penalties relative to pure systems through optimized regeneration and reduced compression requirements. These improvements stem from the temperature glide matching heat source profiles, minimizing losses during CO2 desorption. For example, blended pilots integrated with oxyfuel processes report regeneration energies as low as 2.2 GJ/t CO2, a notable reduction from benchmark MEA systems at 3.5-4 GJ/t CO2. Notable projects include NET Power's Allam Cycle demonstration , operational since 2018 with grid synchronization achieved in 2021, which incorporates zeotropic oxyfuel mixtures to enhance CO2 capture in supercritical cycles, achieving near-zero emissions with integrated sequestration. This oxy-combustion approach uses zeotropic additives in the to improve efficiency and CO2 purity (>99%), demonstrating scalability in natural gas-fired . The first utility-scale is under as of 2025, expected to come in 2027. Looking ahead, the integration of zeotropic mixtures with Organic Rankine Cycles (ORCs) holds substantial potential for utilizing in (CCS) systems, recovering low-grade from compression and regeneration steps to boost overall plant efficiency. Zeotropic ORC working fluids, such as blends, provide better thermal matching during and , yielding 5-10% higher power output than pure fluids in CCS-integrated or IGCC plants. This synergy reduces the net energy penalty of CCS by repurposing otherwise wasted heat, supporting broader deployment in industrial-scale applications.

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